Using ${index()} inside ${concat()} fails with "expected identifier or string literal" error #138181
Description
I tried this code:
#![feature(macro_metavar_expr)] #![feature(macro_metavar_expr_concat)] #![allow(dead_code)] macro_rules! state_num { ($($state:ident),+) => { enum StateNum { $(${concat(S, ${index()})} ${ignore($state)}),+ } }; } state_num!(A, B, C); fn main() { let _s = StateNum::S1; }I expected to see this happen: it compiling successfully.
Instead, this happened:
error: expected identifier or string literal --> src/main.rs:8:28 | 8 | $(${concat(S, ${index()})} ${ignore($state)}),+ | ^^^^^^^^^ error: expected identifier, found `$` --> src/main.rs:8:15 | 7 | enum StateNum { | -------- while parsing this enum 8 | $(${concat(S, ${index()})} ${ignore($state)}),+ | ^ expected identifier ... 13 | state_num!(A, B, C); | ------------------- in this macro invocation | = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }` = note: this error originates in the macro `state_num` (in Nightly builds, run with -Z macro-backtrace for more info) error[E0433]: failed to resolve: use of undeclared type `StateNum` --> src/main.rs:16:14 | 16 | let _s = StateNum::S1; | ^^^^^^^^ use of undeclared type `StateNum` For more information about this error, try `rustc --explain E0433`. Meta
rustc --version --verbose:
rustc 1.87.0-nightly (b74da9613 2025-03-06) binary: rustc commit-hash: b74da9613a8cb5ba67a985f71325be0b7b16c0dd commit-date: 2025-03-06 host: x86_64-unknown-linux-gnu release: 1.87.0-nightly LLVM version: 20.1.0